Table 9-2. Request Type Support (continued)
Event that generates Burst Request
Event that generates Single Request
Peripheral
RX FIFO Level (fixed at 4)
RX FIFO Not Empty
SSI RX
TX FIFO Level (configurable)
TX FIFO Not Full
UART TX
RX FIFO Level (configurable)
RX FIFO Not Empty
UART RX
9.2.4.1
Single Request
When a single request is detected, and not a burst request, the μDMA controller transfers one item
and then stops to wait for another request.
9.2.4.2
Burst Request
When a burst request is detected, the μDMA controller transfers the number of items that is the
lesser of the arbitration size or the number of items remaining in the transfer. Therefore, the arbitration
size should be the same as the number of data items that the peripheral can accommodate when
making a burst request. For example, the UART generates a burst request based on the FIFO trigger
level. In this case, the arbitration size should be set to the amount of data that the FIFO can transfer
when the trigger level is reached. A burst transfer runs to completion once it is started, and cannot
be interrupted, even by a higher priority channel. Burst transfers complete in a shorter time than the
same number of non-burst transfers.
It may be desirable to use only burst transfers and not allow single transfers. For example, perhaps
the nature of the data is such that it only makes sense when transferred together as a single unit
rather than one piece at a time. The single request can be disabled by using the
DMA Channel
Useburst Set (DMAUSEBURSTSET)
register. By setting the bit for a channel in this register, the
μDMA controller only responds to burst requests for that channel.
9.2.5
Channel Configuration
The μDMA controller uses an area of system memory to store a set of channel control structures
in a table. The control table may have one or two entries for each μDMA channel. Each entry in the
table structure contains source and destination pointers, transfer size, and transfer mode. The
control table can be located anywhere in system memory, but it must be contiguous and aligned on
a 1024-byte boundary.
Table 9-3 on page 684 shows the layout in memory of the channel control table. Each channel may
have one or two control structures in the control table: a primary control structure and an optional
alternate control structure. The table is organized so that all of the primary entries are in the first
half of the table, and all the alternate structures are in the second half of the table. The primary entry
is used for simple transfer modes where transfers can be reconfigured and restarted after each
transfer is complete. In this case, the alternate control structures are not used and therefore only
the first half of the table must be allocated in memory; the second half of the control table is not
necessary, and that memory can be used for something else. If a more complex transfer mode is
used such as ping-pong or scatter-gather, then the alternate control structure is also used and
memory space should be allocated for the entire table.
Any unused memory in the control table may be used by the application. This includes the control
structures for any channels that are unused by the application as well as the unused control word
for each channel.
683
June 18, 2014
Texas Instruments-Production Data
Tiva
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TM4C1294NCPDT Microcontroller